Non-linear capacitors, i.e. capacitors whose capacitance depends on the applied voltage in a nonlinear fashion, are useful, for example, for signal mixing, i.e. for generating an output signal from the circuit that is at a different frequency from that of any of the signals applied to the input of the circuit. Such circuits are useful in many communication applications, where it is desirable that the response from the circuit is at a different frequency than the input signal.